Presently electricity produced by the power plant is not stored in the transmission system, but is rather generated at the same time as it is consumed. Consequently, it is necessary to uphold reserves at all times in order to respond instantly to unexpected changes in consumption. These are especially short term load variations, also referred to as ripples, being fluctuations in the order of minutes or even less than one minute. The reserves are activated within a few seconds in order to compensate for load variations. Other options to compensate for load variations are to adjust the energy production such as through fuel adjustment, throttling steam valve or condensate flow control. All of these lead to reduced efficiency of power generation, and it has been estimated that the annual global costs of operational losses in the power industry amounts to more than 14.000.000 .
One document U.S. Pat. No. 7,123,994 describes a power consumption management method of managing energy consumed by a group of energy consuming devices. The energy consuming devices exchange messages according to an energy management control protocol via a communication media. The energy management control protocol comprises an energy booking message type for announcing future energy consumption, an energy reduction indication message type for announcing possible reduction of energy consumption and a granting message type for granting an energy booking message and/or an energy reduction indication. The energy consuming devices negotiate their energy consumption by means of the messages exchanged according to the energy management control protocol and control their energy consumption according to the result of this negotiation.
When receiving a request to decrease power consumption, the energy consuming devices determine possible reduction of energy consumption, assign priorities to these possibilities and announce these possibilities via an indication message to the other ones of the energy consuming devices.
Among the examples is the operation of a compressor or a refrigerator or freezer that might be interrupted for a specific time, if the cooling temperature of the refrigerator or freezer does not exceed a predefined critical temperature. If the cooling temperature exceeds such temperature, the control units assign a correspondingly high priority to the booking and indication messages.
Another example is a tumble-drier that may be interrupted within predefined time frames without any operational drawback.
A further example is a washing machine that can stop at stage of the washing program and may delay the start of the spin-dry-program.
Further, it is possible that different device priorities are assigned in the scheduling rules to different energy consuming devices. Such device priorities reflect the priorities chosen by the customer for its different energy consuming devices.
The system thus includes knowledge of the coming energy consumption of the devices of the group by the use of a protocol, and does not relate to short term non-predictable ripples.
Another document, WO06128709 does relate to a grid responsive control device, describing a load control device which is responsive to a physical variable representing the balance between load and generation on an electricity grid. The control device varies the energy consumption of the load based on the current value of the physical variable of the grid relative to a central value of that physical variable, which is derived from past readings of the physical variable of the grid. The grid responsive control device also takes into account the time since the load last varied its energy consumption in determining whether or not the grid variable load control should be provided.
Further, the document mentions that the primary function of the load is to maintain the physical variable within specified control limits. The loads will generally operate on a duty cycle, usually with a period in which the load is on and with a period in which the load is off.
The document, however, describes a method relating to a history of measurements of the individual devices, and is therefore not very suitable for short term responses to ripples on the power grid.